Conventional steam crackers are known as an effective tool for cracking high-quality feedstocks that contain a large fraction of volatile hydrocarbons, such as ethane, gas oil, and naphtha. Regenerative pyrolysis reactors are also known and conventionally used for converting or cracking and to execute cyclic, high temperature chemistry such as those reactions that may be performed at temperatures higher than can suitably be performed in conventional steam crackers. Regenerative reactor cycles typically are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Symmetric cycles are typically used for relatively mild exothermic chemistry, examples being regenerative thermal oxidation (“RTO”) and autothermal reforming (“ATR”). Asymmetric cycles are typically used to execute endothermic chemistry, and the desired endothermic chemistry is paired with a different chemistry that is exothermic (typically combustion) to provide heat of reaction for the endothermic reaction. Examples of asymmetric cycles are Wulff cracking, Pressure Swing Reforming, and other regenerative pyrolysis reactor processes. Regenerative pyrolysis reactors are generally known in the art as being capable of converting or cracking hydrocarbons. However, they have not achieved commercial or widespread use for hydrocarbon conversion, due at least in part to the fact that they have not scaled well to an economical size. This failure is in large part due to the inability of the equipment to adequately control and contend with the very high temperatures and the way that fuel and oxidant are combined during the regeneration or heating stage of the process. The high temperatures are difficult to position and contain for extended periods of time and lead to premature equipment failure. A solution was proposed in a patent application filed in the U.S.P.T.O., on Dec. 21, 2006, entitled “Methane Conversion to Higher Hydrocarbons,” (2006EM215US), Ser. No. 11/643,541 related primarily to methane feedstocks for pyrolysis systems, utilizing an inventive deferred combustion process with a reverse-flow reactor system. U.S. Patent Application Ser. No. 60/933,044, filed Jun. 4, 2007 (2007EM150PRV), entitled “Pyrolysis Reactor Conversion of Hydrocarbon Feedstocks into Higher Value Hydrocarbons,” teaches removing nonvolatiles from a pyrolysis feed prior to its introduction to the pyrolysis reactor, wherein fuel and oxygen are added in a first reactor to provide heat to a second reactor in which a hydrocarbon feed is pyrolyzed. U.S. Application Ser. No. 60/933,011, filed Jun. 4, 2007 (2007EM151PRV), entitled “Conversion of Co-Fed Methane and Hydrocarbon Feedstocks into Higher Value Hydrocarbons,” teaches removing nonvolatiles from a pyrolysis feed to provide a vapor phase which is fed with methane to a pyrolysis reactor system to provide acetylene. All of the foregoing U.S. patent applications are incorporated herein by reference in their entirety.
As with steam crackers, regenerative pyrolysis reactors are well suited for volatized or volatizable feedstocks that are substantially free of nonvolatile components, such as metals and other residual or nonvolatizable components, which would otherwise lay down and build up in the reactor as ash. Pyrolysis reactors typically operate at higher temperatures than steam crackers.
Typically, regenerative reactors include a reactor bed or zone, typically comprising some type of refractory material, where the reaction takes place within the reactor system. Conventional regenerative reactors typically deliver a stream of fuel, oxidant, or a supplemental amount of one of these reactants, directly to a location somewhere within the flow path of the reactor bed. The delivered reactants then are caused to exothermically react therein and heat the reactor media or bed. Thereafter, the reacted reactants are exhausted and a pyrolysis feedstock, such as a hydrocarbon feed stream, preferably vaporized, is introduced into the heated region of the reactor media or bed, and exposed to the heated media to cause heating and pyrolysis of the reactor feedstock into a pyrolyzed reactor feed. The pyrolyzed reactor feed is then removed from the reaction area of the reactor and quenched or cooled, such as in a quench region of the reactor system, to halt the pyrolysis reaction and yield a pyrolysis product. Such an arrangement requires a dedicated fuel to heat the reactor bed, which fuel is separate from the feed which is to be pyrolyzed and so introduces additional complexity to the operation.
“On the Mechanism of Carbonisation of Benzene, Acetylene and Diacetylene at 1200° C.”, Kinney, C. R. & Slysh, R. S. 1960 Proc. 4th Carbon Conference, Pergamon Press, at p. 301 et seq., teaches conversion of methane at reasonable yields to acetylene and ethylene through methyl radical and hydrogen radical intermediates at 2000° C. Heavier hydrocarbons can be converted to acetylene and syngas at temperatures above 2000° C., but if their hydrogen content is low, the reaction yields significant coke. Benzene with hydrogen content of less than 8% yields only 18 wt % C2, and 70 wt % carbon at 70% conversion, 1200° C., and 56 milliseconds. The reaction is preferentially carried out in the presence of hydrogen (hydropyrolysis) to further reduce soot formation for aromatic feeds.
Garifzyanova and Garifzyanov report in the “Pyrolysis of Resid by the Plasma Chemical Method,” Chem. Tech. Fuels & Oils 42, p. 172 (2006) that pyrolyzing vacuum resid feed containing 10% atomic hydrogen content with a hydrogen methane plasma acting as a hydrogen donor improves C2 yields in the product. The plasma generates hydrogen radicals.
Heavy hydrocarbon liquids can be used in feedstocks for thermal cracking, as well as solid hydrocarbonaceous materials. U.S. Pat. No. 4,536,603 to Sprouse et al., whose contents are incorporated herein by reference in their entirety, discloses a process for reacting coal with a hot gas stream to produce acetylene by reacting fuel, oxygen and steam to provide a hot gas stream that is accelerated and impinged upon a stream of particulate bituminous or subbituminous coal, and the resulting mixture decelerated to produce a product stream. The acceleration and deceleration can be carried out in a convergent-divergent nozzle. U.S. Pat. No. 4,256,565 to Friedman et al., whose contents are incorporated herein by reference in their entirety, teaches the production of olefins from low hydrogen content heavy hydrocarbons containing aromatics such as petroleum residua, asphalts and heavy gas oils. Hydrogen and oxygen are reacted in a first reaction zone to provide a heated gas stream of hydrogen and water at 1000° to 2000° C. which is reacted with sprayed hydrocarbon feed to provide within 2 milliseconds a reaction mixture of 800° to 1800° C. which is maintained for 1 to 10 milliseconds to form enhanced yields of olefins and then quenched. U.S. Pat. No. 6,365,792 to Stapf et al., whose contents are incorporated herein by reference in their entirety, discloses the preparation of acetylene and synthesis gas by thermal treatment of a starting mixture, e.g., methane, higher hydrocarbons and molecular oxygen which mixture is heated to a maximum of 1400° C., brought to reaction in a reactor and cooled, with less solid carbon being formed. U.S. Pat. No. 4,264,435 to Read et al., whose contents are incorporated herein by reference in their entirety, teaches cracking crude oil in an adiabatic reactor utilizing a partial combustion zone generating hydrogen, carbon monoxide, carbon dioxide, and water. Injection of superheated or shift steam into the burner or combustion gases produces more carbon dioxide and hydrogen by the shift reaction, and subsequent injection of crude oil enhances olefins and aromatics production while minimizing coking. Heavy oils generated by the process can be used as fuel for the partial combustion burner. Such low hydrogen content hydrocarbon materials when converted at reasonable yields to acetylene and ethylene at temperatures below 1400° C. result in significant coke formation.
U.S. Pat. No. 5,068,486 to Han et al. reveals a partial oxidation process that operates at very high pressure (20-100 atm), necessitating very high compression costs. The conversion of methane, which is the hydrocarbon feed, is reported as 12.6%, with hydrocarbon selectivity of 32%. The overall conversion of methane to ethylene, acetylene, and propane were 1.4%, 0.4% and 0.1%, respectively. U.S. Pat. Nos. 5,886,056 and 5,935,489 to Hershkowitz et al. teach a multi-nozzle design for feeding a partial oxidation reactor. The multiple nozzles allow introduction of a pre-mix of oxidant and fuel at the burner face so that these gases are premixed and of uniform composition.
It would be highly desirable to provide a process which is suited not only to pyrolysis of heavy liquid feeds containing non-volatiles, but also to other low hydrogen content hydrocarbons including hydrocarbonaceous solids, e.g., coal. Moreover, it would be desirable to carry out such a process utilizing the hydrocarbonaceous feed itself or a component thereof as a source of heat to effect the pyrolysis, in such a way as to minimize coke and tar formation.